The present invention relates generally to adjustable speed drive (ASD) circuits and, more particularly, to a system and method for intelligent circuit breaking for ASD circuits to protect the ASD circuits from being damaged by a fault condition.
One type of system commonly used in industry that performs power conversion is an ASD circuit, also known as a variable frequency drive (VFD) circuit. An ASD is an industrial control device that provides for variable frequency, variable voltage operation of a driven system, such as an AC induction motor. A typical ASD circuit 10 is illustrated in FIG. 1. ASD or VFD circuit 10 includes an electromagnetic interference (EMI) filter 12 provided between a drive input 14 and input rectifier terminals 16. ASD circuit 10 also includes a pre-charge circuit 18 including three pre-charge relays RY1, RY2, RY3, one diode 20 and one gate resistor 22 in series with each pre-charge relay RY1, RY2, RY3, and one pre-charge resistor 24 in series with all three pre-charge relays RY1, RY2, RY3.
ASD circuit 10 further includes a rectifier bridge 26 for converting an AC input power into a DC power. Rectifier bridge 26 includes three silicon-controlled rectifiers (SCRs) 28 that are each coupled in series with one diode 30 and electrically connected to one gate resistor 22 of pre-charge circuit 18. The ASD circuit 10 additionally includes a DC link 32 that receives the DC power from rectifier bridge 26; a DC link capacitor bank 34 having two capacitors 36, 38 across DC link 32; two inductors 40 coupled in series with and on either side of the rectifier bridge 26 on DC link 32; an inverter 42 in parallel with the DC link capacitor bank 34; and an output 44 coupled to the inverter 42.
Pre-charge circuit 18 of ASD 10 operates to control and limit inrush current into DC link capacitor bank 34 during power up (when drive input 14 is initially coupled to an AC input power). Pre-charge relays RY1, RY2, RY3 of pre-charge circuit 18 may be controlled by a controller (not shown) so that when pre-charge relays RY1, RY2, RY3 are in an “OFF” position, pre-charge relays RY1, RY2, RY3 allow current to flow through the pre-charge resistor 24 to pre-charge the DC link capacitor bank 34, and when pre-charge relays RY1, RY2, RY3 are in an “ON” position, pre-charge relays RY1, RY2, RY3 allow current to flow through corresponding gate resistors 22 to power on SCRs 28 of rectifier bridge 26. However, ASD circuit 10 is not protected against any fault conditions that could damage ASD circuit 10.
ASD circuits may be damaged by various internal and external fault conditions. One internal fault condition that occurs in ASD circuits is a short circuit across a DC capacitor in a DC capacitor bank, such as DC capacitor bank 34. A capacitor may develop a short circuit for a variety of reasons including, for example, high temperatures, mechanical damage, aging, or a power disturbance such as a power surge or a voltage transient. In any case, a short circuit across a capacitor in an ASD circuit will cause the ASD circuit to fail. If power is allowed to continue to flow to the capacitor bank when one capacitor has a short circuit, an overvoltage condition will exist on the non-shorted capacitor. This overvoltage condition may, in extreme cases, cause the non-shorted capacitor to catch on fire and possibly explode.
As constructed in the embodiment of FIG. 1, ASD circuit 10 cannot prevent power from continuing to flow to a shorted capacitor in DC capacitor bank 34. That is, as SCRs 28 and diodes 30 of rectifier bridge 26 cannot be controlled to turn off, power can continue to pass through rectifier bridge 26 to DC capacitor bank 34 while a capacitor is shorted. The overvoltage condition on the non-shorted capacitor will cause ASD circuit 10 to be damaged, such that the damaged ASD circuit 10 will not be useable and a user will have to make costly repairs to the ASD circuit or replace the ASD circuit. The user will incur additional economic losses during the downtime period when ASD circuit 10 is not in use.
It would therefore be desirable to provide a system and method for protecting ASD circuits against fault conditions that could damage the ASD circuits.